1. Field of the Invention
The present invention relates to improved carbon black concentrates and black-filled polyolefin extrusion compositions prepared therefrom.
2. Description of the Prior Art
The use of carbon black in thermoplastic compositions to impart ultraviolet (UV) light stabilization is well known. This approach is widely used with polyolefins utilized for outdoor applications such as for electrical and telecommunication wire and cable, agricultural film, geomembranes, plastic pipe and the like. Typically for these applications, loadings of up to about 2.5 weight percent carbon black are used.
While the carbon black provides some degree of protection against thermal and oxidative degradation, it is still generally considered advantageous to include one or more conventional stabilizers, such as hindered phenols and phosphites. Even with the inclusion of additional stabilizers, such as hindered phenols and phosphites, significant degradation can occur during stoppages such as when lines are converted to run different products. Such stoppages can take up to several hours during which time extrusion resin remains in the processing equipment. Significant localized degradation can occur during such stoppages so that when the line is restarted significant gel particles are present in the extrudate producing a rough and undesirable product. The problem can be so severe so that in some instances it is necessary to purge the entire system using a purge resin before the normal extrusion coating operation can be resumed.
The ability to provide effective UV protection depends not only on the type and amount of black but also on the dispersion quality of the carbon black in the polyolefin. It is generally recognized that smaller particle size carbon blacks (higher surface area) increase UV protection and absorption. On the other hand, these smaller particle size blacks are more difficult to disperse in plastic materials. Therefore, even when utilizing small particle size blacks, care must be taken to disperse the carbon black uniformly within the polymer matrix if optimum UV stabilization is to be achieved.
The influence of carbon black dispersion on UV stability and weathering performance is discussed in an article by C. VanBellingen and J. Accorsi, International Wire & Cable Symposium Proceedings, 1999, pp 815-822, entitled "The Impact of Carbon Black Morphology and Dispersion on the Weatherability of Polyethylene." The authors conclude that weatherability and retention of physical properties directly depend on dispersion quality since poor dispersion results in larger agglomerates which have a tendency to scatter UV light rather than absorb it.
To most effectively incorporate and disperse carbon blacks in polyolefin resins at the levels utilized for UV stabilization, masterbatch procedures are most commonly used. These procedures involve first preparing a concentrate, i.e., masterbatch, having a relatively high concentration of the carbon black using a resin in which the carbon black is readily dispersed and which is compatible with the "end-use" resin. Other additives, such as antioxidants and the like, may be included in the masterbatch if desired. Typically, the resin used for the concentrate will be the same as the end-use resin or will be a resin from the same polymer family but having better processability than the end-use resin. The concentrate is then "let-down" into the end-use resin to achieve the desired carbon black loading in the final product. This technique eliminates the need for the processor to handle messy dry carbon black powder and minimizes the processing/equipment required in order to achieve satisfactory dispersion of the carbon black in the end-use resin.
Benzimidazole compounds are known stabilizers for thermoplastic resins such as polyethylenes and polypropylene. U.S. Pat. No. 3,218,276 discloses the use of alkyl benzimidazoles to stabilize fiber-forming polyolefins. Polypropylene fiber-forming compositions containing 0.2 to 2.0 percent benzimidazole with other conventional additives are disclosed.
U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working and zinc mercaptobenzimidazole is specifically mentioned.
The combination of hindered phenols with various zinc salts of mercapto compounds to stabilize polyolefins is taught in U.S. Pat. Nos. 4,260,661; 4,693,937; 4,797,323 and 4,824,883. For example, combinations of IRGANOX 1010 with the zinc salts of 2-mercaptobenzimidazole and 2-mercaptotolylimidazole are disclosed.
Other references which disclose benzimidazole stabilizers for polymeric materials include U.S. Pat. Nos. 4,459,380; 4,808,643 and 5,196,462.